1. Field of the Invention
The present invention concerns improved gray component replacement techniques for color halftone processing.
2. Description of the Related Art
Gray component replacement techniques (or undercolor removal techniques) in use in today's digital halftoning processes are inherited from traditional photolithographic gray component replacement techniques. Specifically, the gray component replacement step is generally performed as an independent step on continuous tone image values at the color separation stage, before sending the separated color planes toward a print driver. This conventional technique is illustrated in more detail in FIG. 1.
Thus, FIG. 1 shows a conventional digital halftoning process in which a continuous tone RGB image (that is, an image having continuous tone red, green and blue image values) is rendered into a halftone image for printout using CMYK (that is, cyan, magenta, yellow and black) color components. The continuous tone RGB values are received and converted to continuous tone CMY values. Gray component replacement then proceeds in which a continuous tone K value is assigned to the minimum of the CMY continuous tone values, multiplied by some percentage n. The value of n specifies the degree of gray component replacement, with typical values being 60 or 70%; 100% signifies complete undercolor removal. The CMY continuous tone values are then adjusted by subtracting out the continuous tone K value. Thereafter, the continuous tone CMYK values are converted to halftone C.sub.OUT, M.sub.OUT, Y.sub.OUT and K.sub.OUT values. Since the digital halftoning process contemplated herein applies to binary printout devices, such as color laser beam printers or color bubble jet printers which either print or do not print a color dot at each pixel, each of the C.sub.OUT, M.sub.OUT, Y.sub.OUT and K.sub.OUT values is represented by only a single bit. CMYK color component values are then printed at each pixel in accordance with the C.sub.OUT, M.sub.OUT, Y.sub.OUT and K.sub.OUT values for each such pixel.
The FIG. 1 approach in which gray component replacement is based on continuous tone values works well for continuous tone printers (i.e., printers that can apply continuously variable amounts of colorant to each pixel). The FIG. 1 approach also works well in traditional offset printing-based separation systems, where precise positioning of each printed color layer cannot be ensured but where it is justifiable to ignore relative phase shift between color layers since such shifts do not introduce any significant color shifts. However, the FIG. 1 approach is ill-suited for binary-type printers such as color bubble jet printers which either print or do not print a color dot at each pixel, and which print each pixel with great positional accuracy. This is because halftoning is performed after gray component replacement, resulting in a situation in which it is not possible to predict exactly where a black pixel will be printed with respect to the CMY pixels that it is supposed to replace. Since each pixel is printed with great positional accuracy, this situation can result in "holes" in the printed image, the holes being formed when CMY colorant dots for a pixel have been replaced by a black colorant dot, but the black colorant dot is printed at an adjacent pixel and not at the pixel where the CMY colorants have been replaced.
There thus exists a need for improved gray color replacement techniques for use in digital halftoning.